Laser vibrometry: pseudo-vibrations

The application of Laser Doppler Velocimetry (LDV) to the measurement of normal-to-surface vibration of a solid surface is now established as a technique complementary to the use of an accelerometer. Several practical systems have been developed and a number are now commercially available. Each velocirneter relies on the same principle of operation, namely the detection of a Doppler shift, fD, in the light scattered from a vibrating target. Fig. 1 shows a typical vibrometer arrangement. Since the photodetector cannot respond quickly enough to detect the light frequency directly, scattered light from the vibrating surface is mixed with a reference beam and heterodyned on the detector surface. In addition, in order to resolve the sign of the vibration velocity, it is necessary to pre-shift the reference beam by a known amount, fR, resulting in an optical beat at the detector of frequency (fR ± fD). An appropriate Doppler signal processor then demodulates the detector signal to produce a time-resolved analogue of the target vibration velocity (in the direction of the incident beam). Systems differ in the method adopted to produce the reference beam frequency shift. Bragg cells [1], diffraction gratings [2] rotating scattering discs [3] and frequency modulation of the laser beam itself [4] have all been used successfully... (continues)

On laser vibrometry of rotating targets: effects of torsional and in-plane motion

Vibration measurements on rotating surfaces are often referred to in ·the commercial literature as a major application of laser Doppler vibration transducers. This paper examines such use of these instruments and shows how the presence of a velocity component due to the rotation itself leads to spurious measurement dependence on both torsional vibration and motion perpendicular to the line of incidence of the laser beam. In addition, the scale of this dependence increases with both rotation speed and perpendicular distance between the line of incidence and a parallel line through the centre of rotation. These phenomena are investigated theoretically and excellent agreement is found when compared with experimental data. Two solutions are suggested; the first involves careful alignment of the laser beam whereas the second requires two simultaneous, orthogonal measurements to be made. If neither method is adopted it is entirely conceivable that the intended solid body vibration measurement may be masked at many frequencies of interest